The present invention relates to a ferroelectric memory including a plurality of ferroelectric capacitors, each of which has a lower electrode, a capacitor dielectric film of a ferroelectric film and an upper electrode formed successively on a semiconductor substrate, arranged in a matrix shape along a word line direction and a bit line direction, and a method for fabricating the ferroelectric memory.
Recently, a nonvolatile ferroelectric memory including a capacitor dielectric film made from a ferroelectric material having a hysteresis characteristic, such as SrBi2Ta2O9 (hereinafter referred to as SBT) or Pb(Zr, Ti)O3 (hereinafter referred to as PZT), has been developed. The ferroelectric material such as SBT or PZT used in this ferroelectric memory is a ferroelectric oxide.
Therefore, the ferroelectric oxide is reduced if it is exposed to a reducing atmosphere or a hydrogen atmosphere in particular during annealing performed in an atmosphere including hydrogen for securing the characteristic of a MOS transistor formed on a semiconductor substrate after forming aluminum wires above a plurality of ferroelectric capacitors with an interlayer insulating film sandwiched therebetween or during CVD performed for filling a tungsten film in a contact hole with an aspect ratio sufficiently high for reducing the size of the semiconductor memory. As a result of the reduction, the crystal composition of the ferroelectric oxide is broken, so that the insulating property of the capacitor dielectric film or the characteristic of the ferroelectric oxide can be largely degraded.
Accordingly, in order to prevent the capacitor dielectric film of the ferroelectric capacitor from being reduced through the exposure to hydrogen even when the ferroelectric capacitor is subjected to annealing performed in a hydrogen atmosphere after forming it, a hydrogen barrier film for preventing hydrogen from invading the capacitor dielectric film is formed over the ferroelectric capacitor.
In the case where the hydrogen barrier film is formed between the ferroelectric capacitor and an interlayer insulating film formed thereon, the area of the hydrogen barrier film should be larger than that of the ferroelectric capacitor by at least several xcexc m for preventing the hydrogen from invading in a horizontal direction. Also, since the hydrogen barrier film is formed also on a contact plug buried in the interlayer insulating film, if the contact plug is made from a tungsten film formed by the CVD, the effect of the hydrogen barrier film to prevent the hydrogen invasion of the capacitor dielectric film is lowered.
In particular, the area of the ferroelectric capacitor has recently been reduced (to 1 xcexc m2 or less) due to the size reduction of the ferroelectric memory, and for the above-described reason, the hydrogen invasion of the capacitor dielectric film cannot be definitely prevented merely by covering the ferroelectric capacitor with the hydrogen barrier film.
As a countermeasure, Japanese Laid-Open Patent Publication No. 11-135736 has proposed a ferroelectric memory having a structure as shown in FIG. 6.
Now, the ferroelectric memory of FIG. 6 will be described as a conventional example.
An isolation region 11 and impurity diffusion layers 12 serving as a source and a drain are formed in surface portions of a silicon substrate 10. A gate electrode 13 is formed on the silicon substrate 10 with a gate insulating film sandwiched therebetween in a portion between the impurity diffusion layers 12. The gate electrode 13 and the impurity diffusion layers 12 together form a field effect transistor.
A first interlayer insulating film 14 is formed on the field effect transistor and the isolation region 11, and a first insulating hydrogen barrier film 15 is formed on the first interlayer insulating film 14 above the isolation region 11. A ferroelectric capacitor including a lower electrode 16, a capacitor dielectric film 17 of a ferroelectric film and an upper electrode 18 is formed on the first insulating hydrogen barrier film 15. A conducting hydrogen barrier film 19 is formed on the upper electrode 18, and a second insulating hydrogen barrier film 20 is formed on the top face of the conducting hydrogen barrier film 19 and the side faces of the lower electrode 16, the capacitor dielectric film 17 and the upper electrode 18. Thus, the ferroelectric capacitor is completely covered with the first insulating hydrogen barrier film 15, the conducting hydrogen barrier film 19 and the second insulating hydrogen barrier film 20.
A second interlayer insulating film 21 is formed on the first interlayer insulating film 14 and the second insulating hydrogen barrier film 20. A metal wire 22 is formed on the second interlayer insulating film 21, and the metal wire 22 is connected to a contact plug 23 buried in the first interlayer insulating film 14 and the second interlayer insulating film 21.
Since the ferroelectric capacitor is completely covered with the first insulating hydrogen barrier film 15, the conducting hydrogen barrier film 19 and the second insulating hydrogen barrier film 20 as described above, the hydrogen invasion of the capacitor dielectric film 17 can be prevented.
In this conventional ferroelectric memory, however, a side portion of the second insulating hydrogen barrier film 20 may disappear or be reduced in its thickness due to mask shift occurring in patterning the second insulating hydrogen barrier film 20.
Therefore, it is necessary to increase the thickness of the second insulating hydrogen barrier film 20 and to increase the margin of a mask used for pattering the second insulating hydrogen barrier film 20.
As a result, it is necessary to increase the intervals between ferroelectric capacitors, which disadvantageously makes it difficult to reduce the size of the ferroelectric memory.
In consideration of the aforementioned conventional problem, an object of the invention is definitely preventing hydrogen from invading a capacitor dielectric film of a ferroelectric capacitor as well as reducing the size of a ferroelectric memory.
In order to achieve the object, the ferroelectric memory of this invention comprises a plurality of ferroelectric capacitors, each of which includes a lower electrode, a capacitor dielectric film of a ferroelectric film and an upper electrode successively formed on an interlayer insulating film on a semiconductor substrate, the plurality of ferroelectric capacitors being arranged along a word line direction and a bit line direction, a first insulating hydrogen barrier film is filled between the lower electrodes of some ferroelectric capacitors, among the plurality of ferroelectric capacitors, that are arranged along one direction out of the word line direction and the bit line direction, the capacitor dielectric film is formed as a common capacitor dielectric film commonly used by the some ferroelectric capacitors arranged along the one direction and formed on the lower electrodes of the some ferroelectric capacitors arranged along the one direction and on the first insulating hydrogen barrier film, the upper electrode is formed as a common upper electrode commonly used by the some ferroelectric capacitors arranged along the one direction and formed on the common capacitor dielectric film, and a second insulating hydrogen barrier film is formed to cover the common upper electrode.
In the ferroelectric memory of this invention, the first insulating hydrogen barrier film is filled between the lower electrodes of the plural ferroelectric capacitors arranged in one direction out of the word line direction and the bit line direction. Therefore, there is no need to pattern the first insulating hydrogen barrier film in regions between the lower electrodes of the ferroelectric capacitors arranged in the one direction, and hence, there is no need to secure a dimensional margin between the lower electrodes in consideration of shift of a mask used for patterning. As a result, the intervals between the ferroelectric capacitors can be reduced, so as to reduce the area of the memory cell, namely, the area of the ferroelectric memory.
Also, since the second insulating hydrogen barrier film is formed so as to cover the common upper electrode, even when annealing in a hydrogen atmosphere is performed after forming the ferroelectric capacitors, hydrogen invasion of the capacitor dielectric film of the ferroelectric capacitor in a downward direction can be prevented. Thus, the ferroelectric film used for forming the capacitor dielectric film can be prevented from being reduced.
In the ferroelectric memory, the second insulating hydrogen barrier film is preferably formed separately with respect to every capacitor line including some ferroelectric capacitors arranged along the one direction.
Thus, there is also no need to pattern the second insulating hydrogen barrier film in a region between the lower electrodes of the plural ferroelectric capacitors arranged in the one direction, and hence, there is no need to secure a dimensional margin between the lower electrodes in consideration of the shift of a mask used for patterning. Accordingly, the intervals between the ferroelectric capacitors can be reduced, so as to reduce the area of the memory cell, namely, the area of the ferroelectric memory.
In the ferroelectric memory, the second insulating hydrogen barrier film is preferably formed to cover a pair of capacitor lines adjacent to each other along the other direction out of the word line direction and the bit line direction among capacitor lines each including some ferroelectric capacitors arranged along the one direction.
Thus, there is no need to secure a dimensional margin between the pair of capacitor lines each including the plural ferroelectric capacitors arranged in the one direction. Therefore, the interval between the pair of capacitor lines can be reduced, so as to reduce the area of the memory cell array, namely, the area of the ferroelectric memory. Also, since there is a region where no hydrogen barrier film is formed in the vicinity of a selecting transistor of the ferroelectric memory, it is possible to secure a path for hydrogen to diffuse into the selecting transistor during annealing performed in a hydrogen atmosphere for recovering the characteristic of the transistor after forming metal wires.
In the ferroelectric memory, a conducting hydrogen barrier film is preferably. formed between a contact plug formed in the interlayer insulating film and the lower electrodes.
Thus, in the case where annealing in a hydrogen atmosphere is performed after forming the ferroelectric capacitors, hydrogen invasion of the capacitor dielectric film of the ferroelectric capacitors in an upward direction can be prevented. Therefore, the ferroelectric film used for forming the capacitor dielectric film can be prevented from being reduced.
In the case where the ferroelectric memory of this invention includes the conducting hydrogen barrier film, every capacitor line including some ferroelectric capacitors arranged along the one direction is preferably completely covered with the conducting hydrogen barrier film, the first insulating hydrogen barrier film and the second insulating hydrogen barrier film.
Thus, even when annealing is performed in a hydrogen atmosphere after forming the ferroelectric capacitors, the hydrogen invasion of the capacitor dielectric film of the ferroelectric capacitors can be definitely prevented. Accordingly, the ferroelectric film used for forming the capacitor dielectric film can be prevented from being reduced, resulting in definitely preventing degradation of the characteristic of the capacitor dielectric film.
In the ferroelectric memory, a level reducing film for reducing a level difference caused in a peripheral portion of the common upper electrode is preferably formed between the common upper electrode and the second insulating hydrogen barrier film.
Thus, an angular level difference caused in the peripheral portion of the patterned upper electrode can be reduced, so as to improve coverage of the second insulating hydrogen barrier film at the peripheral portion of the upper electrode.
In the ferroelectric memory, the first insulating hydrogen barrier film can be made from a Si3N4 film, a SiON film, an Al2O3 film, a TiO2 film or an oxide or oxide nitrided film of an alloy of Ti and Al.
In the ferroelectric memory, the second insulating hydrogen barrier film can be made from a Si3N4 film, a SiON film, an Al2O3 film, a TiO2 film, a TiN film, an alloy film of Ti and Al, or an oxide, nitride or oxide nitrided film of an alloy of Ti and Al.
In the ferroelectric memory, the conducting hydrogen barrier film can be made from an alloy film of Ti and Al, a nitride or oxide nitrided film of an alloy of Ti and Al, or a TiN film.
The method of this invention for fabricating a ferroelectric memory including a plurality of ferroelectric capacitors, each of which has a lower electrode, a capacitor dielectric film of a ferroelectric film and an upper electrode successively formed on an interlayer insulating film on a semiconductor substrate, the plurality of ferroelectric capacitors being arranged along a bit line direction and a word line direction, comprises the steps of forming the lower electrodes of the plurality of ferroelectric capacitors on the interlayer insulating film; depositing a first insulating hydrogen barrier film on the interlayer insulating film and the lower electrodes and planarizing the first insulating hydrogen barrier film, whereby filling the first insulating hydrogen barrier film between the lower electrodes of some ferroelectric capacitors, among the plurality of ferroelectric capacitors, that are arranged along one direction out of the word line direction and the bit line direction; forming, on the lower electrodes of the some ferroelectric capacitors arranged along the one direction and on the first insulating hydrogen barrier film, the capacitor dielectric film as a common capacitor dielectric film commonly used by the some ferroelectric capacitors arranged along the one direction; forming, on the common capacitor dielectric film, the upper electrode as a common upper electrode commonly used by the some ferroelectric capacitors arranged along the one direction; and forming, on the common upper electrode, a second insulating hydrogen barrier film covering the common upper electrode.
In the method for fabricating a ferroelectric memory of this invention, the first insulating hydrogen barrier film is filled between the lower electrodes of the plural ferroelectric capacitors arranged in one direction. Therefore, there is no need to pattern the first insulating hydrogen barrier film in regions between the lower electrodes of the ferroelectric capacitors arranged in the one direction, and hence, there is no need to secure a dimensional margin between the lower electrodes in consideration of shift of a mask used for patterning. As a result, the intervals between the ferroelectric capacitors can be reduced, so as to reduce the area of the memory cell, namely, the area of the ferroelectric memory.
Also, since the method comprises the step of forming the second insulating hydrogen barrier film so as to cover the common upper electrode, even when annealing in a hydrogen atmosphere is performed after forming the ferroelectric capacitors, hydrogen invasion of the capacitor dielectric film of the ferroelectric capacitors in a downward direction can be prevented. Thus, the ferroelectric film used for forming the capacitor dielectric film can be prevented from being reduced.
In the method for fabricating a ferroelectric memory, the second insulating hydrogen barrier film is preferably formed separately with respect to every capacitor line including some ferroelectric capacitors arranged along the one direction.
Thus, there is also no need to pattern the second insulating hydrogen barrier film in a region between the lower electrodes of the plural ferroelectric capacitors arranged in the one direction, and hence, there is no need to secure a dimensional margin between the lower electrodes in consideration of the shift of a mask used for patterning. Accordingly, the intervals between the ferroelectric capacitors can be reduced, so as to reduce the area of the memory cell, namely, the area of the ferroelectric memory.
In the method for fabricating a ferroelectric memory, the second insulating hydrogen barrier film is preferably formed to cover a pair of capacitor lines adjacent to each other along the other direction out of the word line direction and the bit line direction among capacitor lines each including some ferroelectric capacitors arranged along the one direction.
Thus, there is no need to secure a dimensional margin between the pair of capacitor lines each including the plural ferroelectric capacitors arranged in the one direction. Therefore, the interval between the pair of capacitor lines can be reduced, so as to reduce the area of the memory cell array, namely, the area of the ferroelectric memory. Also, since there is a region where no hydrogen barrier film is formed in the vicinity of a selecting transistor of the ferroelectric memory, it is possible to secure a path for hydrogen to diffuse into the selecting transistor during annealing performed in a hydrogen atmosphere for recovering the characteristic of the transistor after forming metal wires.
The method for fabricating a ferroelectric memory of this invention preferably further comprises, before the step of forming the lower electrodes, a step of forming a conducting hydrogen barrier film between a contact plug formed in the interlayer insulating film and the lower electrodes.
Thus, in the case where annealing in a hydrogen atmosphere is performed after forming the ferroelectric capacitors, hydrogen invasion of the capacitor dielectric film of the ferroelectric capacitors in an upward direction can be prevented. Therefore, the ferroelectric film used for forming the capacitor dielectric film can be prevented from being reduced.
In the case where the method for fabricating a ferroelectric memory of this invention comprises the step of forming a conducting hydrogen barrier film, every capacitor line including some ferroelectric capacitors arranged along the one direction is preferably completely covered with the conducting hydrogen barrier film, the first insulating hydrogen barrier film and the second insulating hydrogen barrier film.
Thus, even when annealing is performed in a hydrogen atmosphere after forming the ferroelectric capacitors, the hydrogen invasion of the capacitor dielectric film of the ferroelectric capacitors can be definitely prevented. Accordingly, the ferroelectric film used for forming the capacitor dielectric film can be prevented from being reduced, resulting in definitely preventing degradation of the characteristic of the capacitor dielectric film.
The method for fabricating a ferroelectric memory of this invention preferably further comprises, between the step of forming the upper electrode and the step of forming a second insulating hydrogen barrier film, a step of forming, between the common upper electrode and the second insulating hydrogen barrier film, a level reducing film for reducing a level difference caused in a peripheral portion of the common upper electrode.
Thus, an angular level difference caused in the peripheral portion of the patterned upper electrode can be reduced, so as to improve coverage of the second insulating hydrogen barrier film at the peripheral portion of the upper electrode.
In the method for fabricating a ferroelectric memory, the first insulating hydrogen barrier film can be made from a Si3N4 film, a SiON film, an Al2O3 film, a TiO2 film or an oxide or oxide nitrided film of an alloy of Ti and Al.
In the method for fabricating a ferroelectric memory, the second insulating hydrogen barrier film can be made from a Si3N4 film, a SiON film, an Al2O3 film, a TiO2 film, a TiN film, an alloy film of Ti and Al, or an oxide, nitride or oxide nitrided film of an alloy of Ti and Al.
In the method for fabricating a ferroelectric memory, the conducting hydrogen barrier film can be made from an alloy film of Ti and Al, a nitride or oxide nitrided film of an alloy of Ti and Al, or a TiN film.